List advantages and disadvantage of a higher water table for
crop growth.

Describe how a producer manages a drainage system during the
year.

Determine the economic feasibility of installing a drainage
control structure.

PRE-TEST

1. Drainage control structures are most suited to fields that
are

hilly.

gently sloping.

level or nearly level.

rocky.

2. Subsurface tile drainage increases

evapotranspiration.

nitrogen leaching from the soil profile.

the lateral flow of soil nutrients.

pesticide loads in groundwater.

3. Raising the water table in a field may decrease all of the
following EXCEPT

earthworm populations.

crop rooting depth.

tile outflow.

denitrification.

4. The hypoxia zone in the Gulf of Mexico is characterized by

low dissolved oxygen in water.

nutrient-rich waters that enhance fish catches

high levels of anhydrous ammonia.

highly elevated surface water temperatures.

5. A soil conducive to plant growth

is a mixture of water, air, organic matter, and minerals.

is saturated with water.

contains more ammonium nitrogen than nitrate nitrogen.

has a water table at least three feet below crop rooting depth.

INTRODUCTION

The installation of subsurface tile drainage that made millions
of Midwest acres more productive is one of the great success stories
of American agriculture. About half of Indiana farm ground benefits
from subsurface drainage. And with the common use of yield monitors
and yield mapping in the past decade, farmers were able to better
quantify the impact of field drainage, renewing interest in field
tile installation (Figure 1).

Unfortunately, it’s been shown that field tiles emptying
into streams (Figure 2) are a major source of excessive nitrogen
and other plant nutrients in those streams, a growing environmental
concern. Also, whereas field tiles provide a big benefit by removing
excess water, retaining some of that water for crop use may be beneficial
some years. A relatively simple solution addressing both of these
concerns is to control the outflow of tiles by the use of water
control structures.

Figure 2. A subsurface field tile draining into a Midwest stream
(NRCS).

HOW DO CONTROL STRUCTURES WORK?

There are different types of structures available to control the
water flowing out of a field tile. One type uses a float that opens
and closes a valve (Figure 3); another is a box containing removable
boards (Figures 4 and 5). Note that the tile is not plugged, but
the level of the water table is adjusted up or down by the addition
or removal of boards that span the structure. The boards are stacked
on their sides and fit into slots along the sides of the structure.

Figure 3. Float-type control structure being installed on an
Indiana farm (Jason Brown).

Control structures are most suited to fields that are level or
nearly level, where slopes don’t exceed 1%. One control structure
might be able to regulate the water table within 1 to 2 feet in
a nearly level field as large as 20 acres. As slopes increase, more
and more control structures are needed to properly regulate water
levels across fields. Also, fields that have a pattern tile system
are better suited, as you can have a greater area of impact across
the field.

Summer and Winter

Figure 4: The boards in the
control structure are installed to raise the drain outlet
after planting (to store water for crops) and after harvest
(to improve water quality).

Spring and Fall

Figure 5 The boards in the control structure are removed a few weeks before field operations such as planting and harvest to allow the field to drain more fully (Purdue University).

For controlling nitrogen pollution the most important factor is
the amount of water leaving the field, but for crop management the
most important factor is the depth of the water table below the
field surface. As a general practice with managed drainage, the
water table is lowered in preparation for spring planting and fall
harvesting operations, and then is allowed to increase in the winter
and summer. Table 1 shows a simplified scheme for how a field might
be managed.

The above management scheme can be modified as conditions warrant.
For example, if spring conditions are extremely dry, a grower may
want to leave the boards installed to retain more water for later
crop use. Or, if summer conditions turn out excessively wet, a grower
may want to remove boards to further lower the water table.

Furthermore, some growers have taken this scheme a step further and
use the system for subirrigation. If an abundant water source is available,
water can be added back into the system for summer crop needs. For
subirrigation to work, there are limitations on how deep and how far
apart tiles are spaced, as well as on the permeability of the underlying
material. Tiles need to be close enough to corn roots to serve as
a source of water. And, if subsoils are too permeable, added water
will be lost to deeper layers.

DRAINAGE AND NITROGEN

Less than 50 years ago corn was generally grown in a rotation
with cereal crops and forage legumes such as alfalfa, red clover,
and sweet clover. Through biological N fixation, the legumes provided
residual N in the soil profile. In addition, most farms were using
animal manures to supplement the nutrient needs of the crop.

Today, most Midwest farmers employ a much less diverse crop rotation.
Livestock operations are concentrated, meaning most farms supply
crop nutrient needs largely through commercial fertilizers, not
animal manure. Commercial nitrogen has been relatively inexpensive,
with economic (and visual) penalties much greater for running short
than applying too much, resulting in applications often exceeding
crop utilization. Compared to past years there’s more nitrogen
in the soil profile, and more is in the inorganic form, which is
more susceptible to leaching.

Farm tiles empty into drainage ditches and streams, and have been
found to be a major source of the increasing nitrogen levels found
in these watercourses. Subsurface drains enhance nutrient leaching
of the soil profile. Nitrate concentrations coming from tile drains
across the Midwest vary according to soil organic matter, crops
and yields, yearly weather variation, fertilizer rates and timing,
tile spacing, and the level of the water table.

Nitrate loads from the Mississippi River to the Gulf of Mexico
have increased 300% since 1970, and non-point sources have contributed
most of the load. More than a million tons of nitrogen makes its
way to the Gulf via the Mississippi River each year. Nitrate over-enrichment
has been shown to contribute to hypoxia (low oxygen levels) and
influences fish populations in the Northern part of the Gulf of
Mexico (Figure 6). In fact, the Gulf of Mexico hypoxic zone is the
second largest area of oxygen depleted waters in the world, and
appears to be growing. The vast majority of the Gulf's commercial
fishery landings come from the area directly affected by the Mississippi
River.

Figure 6. The hypoxia zone, and the major rivers that influence
the Mississippi River watershed (EPA).

The reason hypoxia occurs is that elevated nitrate concentrations
increase the growth of algae. Eventually the algae die, consuming
oxygen as they decompose. The reduced oxygen level can have a significant
impact on the populations of shrimp and many other marine organisms.

Elevated nitrate levels in local streams have not been considered
a major problem for most Midwest water uses. However, new surface
water nutrient criteria being proposed for each EPA ecoregion may
increase nitrate concerns at a local level. These proposed criteria
are 75% lower than current nitrate levels typical in many streams
draining agricultural areas. Once new nutrient standards are implemented,
many water bodies will fail to meet the criteria and will be listed
as impaired, eventually requiring a Total Maximum Daily Load (TMDL).
Widespread implementation of TMDLs for nitrogen will increase the
local demand for agricultural management practices that can substantially
reduce nitrate loads going into surface water.

Controlling tile outflows and soil water tables may be part of the
answer to reducing stream loads of nitrogen. Having a higher water
table through managed drainage means that there is a greater anaerobic
volume in the soil to enhance denitrification, there is less tile
water exiting the system, and there is less of a soil profile that
water moves through to leach. In one eastern Illinois watershed, it
has been estimated that stream nitrate concentrations could be reduced
by 35% if half of the land in the watershed utilized managed drainage.

MANAGED DRAINAGE AND SOIL PROPERTIES

Reducing tile flow and maintaining the water table at a higher
level during certain times of the year is bound to have an impact
on many soil characteristics and subsequent crop responses. Many
of these characteristics remain untested, especially among unique
soil types and in regions with different crops, rainfall, and soil
evaporation/plant transpiration.

In some peat soils, drainage has caused these soils to subside
or settle at a rapid rate. Raising the water table certain times
of the year can reduce oxidation in the soil profile, leading to
preservation of the soil itself. Also, a higher water table means
that lower soil layers contain more water, keeping them more buoyant
under field traffic and less susceptible to compacting, minimizing
settling.

Past studies have shown soil physical properties can be improved
by drainage, raising the question as to whether keeping a higher
water table in place for prolonged periods might be harmful to soil
properties. In one study, there were fewer soil macropores when
the water table was maintained at a higher level.

Earthworms improve soil physical properties, water infiltration,
and drainage. Many farmers are concerned that maintaining higher
water tables might have serious negative effects on earthworm populations
(Figure 7), but at this time there’s not been enough research
to evaluate the effects.

Figure 7. Earthworm burrows may extend several feet down into
the soil profile (USDA).

MANAGED DRAINAGE AND YIELD

Introductory soils courses teach that subsurface tile drains remove
gravitational water. With the water remaining held by capillary
and hygroscopic forces, a soil is said to be at “field capacity”.
Soils at field capacity contain a mix of water, air, mineral, and
organic matter that is conducive to good plant growth. While field
capacity soils are probably close to an ideal moisture status, all
fields will experience a range of soil moisture, and water added
by precipitation will constantly change the situation.

The benefits of storing water to have it available if conditions
turn dry in the summer will need to be weighed against any possible
detrimental effects of a higher water table and a wetter soil profile
earlier in the season. What will be the effects of field operations
such as tillage where the water table may be higher? Will the shallower
crop rooting that may occur with a higher water table actually lead
to greater moisture stress when conditions turn drier? Also, if
there is less N leaving the system by leaching, it reasons that
there might be more available for plant growth. Opposing this, though,
is that higher water tables may cause more nitrogen loss lower in
the soil profile through denitritication.

Although studies combining managed drainage with subirrigation
have shown impressive yield responses, few researchers have measured
yield changes with managed drainage alone (without subirrigation
also). In North Carolina and Louisiana studies, the measured yield
differences were inconsequential. In central Illinois in 2003, nine
of 15 farmers said they had higher yields with drainage management.
More research is needed to better quantify yield effects.

RETURNS FOR MANAGED DRAINAGE

Managed drainage will be most readily adopted by farmers if it
is profitable, that is if it increases yields, decreases costs,
or if it reduces risk. Installing the control structures to manage
drainage will add expense, as will the time and effort to manage
them throughout the year. Thus, to be profitable, the value of additional
yields produced with managed drainage must be greater than the costs
to install and manage the structures.

Cost-share funding may be available for managed drainage. In Indiana,
the USDA NRCS Environmental Quality Incentives Program (EQIP) may
provide funding for Practice Standard 554, Drainage Water Management
and Practice Standard 587, Structure for Water Control. For more
information, talk with your local District Conservationist.

The first step in economic analysis is usually a partial budget.
The increased costs are subtracted from the increased benefits.
The example calculations below can help you decide the profitability
for your own situation. The calculations assume:

At least a portion of the field already has a functioning pattern
tile drainage system.

Managed drainage will provide an increase in corn and soybean
yields.
(Additional research will better substantiate yield responses.)

Change the entries in the yellow boxes to recalculate the worksheet.

First, enter the portions of your field where drainage can
be managed, and estimate the potential yield benefits for corn and
soybeans under drainage management.

Table 2. Managed Drainage Spreadsheet

Example: Managed drainage of a portion of a 100 acre field in a corn/soybean rotation

Corn--------------------------------------

Soybeans ------------------------------

Area

Portion of Field Area

Yield without management, bu/A

Yield with management, bu/A

Production Increase bu/field

Yield without management, bu/A

Yield with management, bu/A

Production Increase bu/field

Managed Drainage Area

225

75

Other Area

0

0

Managed Drainage Acres

15

Current Field Production, bushels

16,000

5,000

Increase with Drainage Management

225

75

Whole Field % Increase with Management

1.41%

1.5%

% Increase per drained acre with management

9.38%

10.00%

Next, enter the cost of purchasing and installing the structure, and its useful life expectancy.

Table 3. Cost to purchase and install structure.

Total Cost of structure and installation

$

EQIP Incentive, $/A (per year, for 3 years)

Before Tax:

Opportunity Cost of Capital

10.00%

Useful Life, years

(Depreciation + Interest)/field

$150.00

(Depreciation + Interest)/managed drainage acre

$10.00

(Depreciation + Interest)/whole field acre

$1.50

Finally, enter grain prices and your cost of labor to maintain the structure.

Table 4. Net annual benefit for a corn/soybean rotation.

Units

$/Unit

Amount

Corn:

Yield Increase, bushels

225

$

$450.00

Increased Production Costs

225

$0.87

$196.26

Soybean:

Yield Increase, bushels

75

$

$375.00

Increased Production Costs

75

$0.48

$35.72

Added Labor, hours

4

$

$40.00

EQIP Incentive

$0.00

Average gross benefit

$256.51

Annual Cost: Depreciation + Interest

$150.00

Pretax Net Benefit/field

$106.51

Pretax Net Benefit/managed drainage acre

$7.10

Pretax Net Benefit/whole field acre

$1.07

Economic assessments are based on the following assumptions:

Marginal costs are charged to production increases. For corn,
these amount to $0.87/bu and include additional N, P, K, lime,
hauling, and drying. For soybeans each additional bushel has a
marginal cost of $0.48 due to additional P, K, lime, and hauling.

The opportunity cost of capital is added to the cost of the
structure at a rate of 10% per year.

It is estimated to take an hour to check a drainage control
structure. If water levels are modified four times per year, then
it will take four hours per year.

The value of the time used to check drainage control structures
is $10 per hour.

Using default values, the example shown in Tables 2, 3, and 4 has controlled drainage
installed on 15 acres in a 100 acre field, with a 15 bushel per
acre corn yield increase and a 5 bushel per acre soybean yield increase.
This is roughly a 10% yield increase on the area affected by the
controlled drainage or an about 1% overall increase for the field.

The structure cost calculations in Table 2 assume that the control
structure and installation cost about $1000, that the useful life
of the structure is 20 years and that the cost of capital is 10%.
Using straight line depreciation and an interest charge on the initial
investment, the annual cost for the field is about $150 ($1000/100
+ $1000*0.1) and the cost per managed drainage acre is about $1.50.

Table 3 contains the partial budget comparing the change in cost
and the change in the value of yield. At $2/bu. corn and $5/bu.
soybeans, average whole field crop value for a corn soybean rotation
is increased by about $412.50 annually. Deducting the cost of labor
for managing drainage, maintaining soil fertility, and extra drying
and hauling, the whole field return over variable costs is increased
by about $257. This is more than enough to cover the annual cost
of the structure. Benefit is estimated as about $7 per managed acre
or about $1 per acre for the whole field.

RECALCULATION: Interactive WorksheetsWhat if a grower is able to secure EQIP funding to cover
part of the cost of purchasing, installing, and managing a drainage
structure? Assume a Cost Share of 50% of structure and installation
cost, and an incentive of $40/A for 3 years for the area
that the control structure influences. How would that change your
calculation?

SUMMARY

Subsurface tile drains have brought millions of acres of otherwise
unfarmable land into production, but are also partially responsible
for polluting streams with nitrogen and other nutrients. Managed
drainage is a concept where tile flow is regulated by installing
structures along tile lines to periodically raise the water table
in the field. For most agricultural situations, water would be allowed
to drain from the field in the spring and fall to allow field operations,
and then be held back in the winter to minimize nitrogen leaching,
and held in the summer to keep more water for crop use. Limited
indications show that managed drainage has the potential to boost
crop yields, especially in drier years, and thereby increase profits.
Many other effects need further study, including the effect of a
raised water table on soil properties, earthworms, and on various
soil types and cropping systems.

FOR MORE INFORMATION

Drainage water management is a new practice in Indiana, and many
questions still need to be answered. For more information, contact
the following resource people, or visit the web sites listed.